May 13 (Reuters) - A 12-year-old boy who went on a shooting rampage at his Nevada middle school in October, killing a teacher and injuring two students before turning the gun on himself, had complained days earlier of being teased by classmates, police said on Tuesday.

The local police chief revealed the complaints, made to a psychotherapist, in a news conference summarising a months-long investigation into the rampage.

Sparks Chief of Police Brian Allen also said the parents of the child, Jose Reyes-Urtiz, would not be charged in connection with the incident.

Reyes-Urtiz used a 9-millimeter semiautomatic handgun his parents kept in the home. But Allen said the parents' contention they did not know their son was aware of the gun was credible, and they took measures to hide the weapon from him.

The investigation yielded several clues into Reyes-Urtiz's actions on the morning of October 21, but provided no single catalyst, Allen said.

"We will never know the complete motive or intent behind this tragic incident, as the only person with the true knowledge felt acting out in violence and taking his own life was the best course of action on that day," Allen said.

"That in itself is a tragedy."

A search of Reyes-Urtiz's computer found that in the six months before the incident there were searches for "Top 10 evil children" and "Super Columbine Massacre Role Playing Game" - a reference to a 1999 school shooting in Colorado.

Three days before the shootings, Reyes-Urtiz told a psychotherapist that schoolmates had called him "gay" and teased him for wetting his pants, apparently after he got water on them, the investigation found.

School records showed Reyes-Urtiz struggled with his verbal skills as early as kindergarten, had been teased by classmates for his speech patterns and may have been autistic, the investigation found.

It turned up two suicide notes left by Reyes-Urtiz. In one, he expressed anger over his treatment in school and wrote of getting revenge. In the other, he apologized to his parents for not being a better son, and said the assault was not the result of his being bullied.

Schoolteacher Michael Landsbery, 45, was shot in the chest and killed in the attack, while two 12-year-old students were shot and injured. Reyes-Urtiz had never taken a class with Landsbery, Allen said. (Editing by Edith Honan and Clarence Fernandez)